Administrivia First pieces of assignment 5 Comments about assignment 5 Assignment 6 Labs 5 and 6

Some bytes e 0a0d 0a1a d c d ccda 79fd 24d e 0a0d 0a1a d c d d4da 61fd 648c

Object oriented programming Figure out characteristics of your data –objects Figure out operations you will want to perform –Methods Modern idea in programming.

Objects Traffic light at intersection involves –Lights in each direction Call them red, yellow and green and not 0,1,2 –Sensors in each direction –Timing (rate of change in each direction) Timings needn’t be the same –Neighboring Lights May affect change as much as sensors

Methods Method of querying color of light Method of changing color of light Method of scheduling a color change later …

What happens to the program? Compiled or interpreted –Eventually it gets translated into machine language If compiled –Can store executable and run again If interpreted –Interpret each time it is executed

What does the compiler do? Identifies variables (need space in RAM) –Uses stores and loads to get values to registers Parses commands –Turns each command into a string of machine language commands Sets things up for execution

Steps in compilation Lexical analysis –Identify all keywords –Identify all operators –Identify all variables –Make everything into tokens Parsing –Turn the tokens into operations –Build a computation tree Code generation –Generate machine code

Lexical analysis Keywords –If … Then.. End If –If.. Then.. Else … End If –Do While … … Loop –Private sub …. –End sub –Dim … as Integer Operators –= (in 2 contexts) If (Light = 0) Light = 1 –+ - * / - also in 2 contexts (unary or binary)

Simple code fragment Sub print_ftoc(low As Integer, high As Integer) Dim fahrenheit As Double, celsius As Double For fahrenheit = low to high celsius = 5 / 9 * (fahrenheit - 32) print fahrenheit, celsius Next fahrenheit End Sub

Simplified code fragment Dim low As Integer, high As Integer Dim fahrenheit As Double, celsius As Double For fahrenheit = low to high celsius = 5 / 9 * (fahrenheit - 32) print fahrenheit, celsius Next fahrenheit End

Code fragment (lex’ed) Dim low As Integer, high As Integer Dim fahrenheit As Double, celsius As Double For fahrenheit = low to high celsius = 5 / 9 * (fahrenheit - 32) print fahrenheit, celsius Next fahrenheit End Keywords, variables, constants, operators, functions, separators

Code fragment (cont.) Dim As Integer, As Integer Dim As Double, As Double For = to = 5 / 9 * ( - 32) print, Next End Replace variables by tags these are really locations in RAM How things are defined determines how much RAM they need how operations on them work

Code fragment (cont.) For = to = 5 / 9 * ( - 32) print, Next The instructions in the loop must be unwound = = 5 / 9 * ( - 32) print, = + 1 If > go back

= = 5 / 9 * ( - 32) print, = + 1 If > go back The unwound loop can be translated into machine language Store 32 in R3 Store 5/9 in R4 Store 1 in R5 Load into R1 ***Store R1 into Load into R2 Subtract R3 from R2 and store in R2 Multiply R4 by R2 and store in R2 Store R2 in Print R1,R2 Add R5 to R1 and store in R1 Store R5 in Load into R6 Subtract R6 from R5 and store in R5 Go back to *** if R6 > 0

Parsing Language is defined by a grammar Grammar is defined by production rules Parsing is done by unwinding

How do we specify a grammar? 2 aspects to a language –Symbols –Rewriting rules Simple language for generating numbers –Symbols Non-terminals –,,, Terminals –

Simple rewriting rules .  + | -  | |   digit   

An example .  + | -  | |   digit    .  …

Simplifying the rules .  + | -  | |   digit    . |  +|-  |  0|1|2|3|4|5|6|7|8|9

Parsing . |  +|-  |  0|1|2|3|4|5|6|7|8|9 What rules were applied to get ?

What about real languages? The complete grammar for C –around 400 lines long –58 tokens (based on keywords) –65 basic productions (each with many options) –Only a few complex situations

Programming language tradeoffs Branching vs. locality –Should the program be in blocks or look like spaghetti Type declarations –If you Dim something as an integer and then try to make it hold a double, what should happen? Verification –How do you tell if your specification is right? –How do you tell if your program meets your specification?

History of Programming Languages Fortran (1954) for scientific Cobol (1959) for business Algol (1958) more universal Fortran Lisp (1958) string/concept oriented APL (1960) formula oriented

History of Programming Languages PL/1 (1964) from Algol + Fortran Basic (1964) for everyone to use Simula (1967) combines with Algol to yield Smalltalk (1969) – object oriented BCPL  B  C (1971) Algol  Pascal (1971)  Modula 1,2,3,

History of Programming Languages C++ (1983) C with object oriented features –Often C is still used Awk (1978)  Perl (1987) report generators –Web programming language Java (1991) object oriented and portable –Web applets, devices Visual Basic(1991) macros and programs –Core of Microsoft systems

What makes a good language Does the task you want Keeps you from making mistakes Supports debugging when you need it Has a strong tool kit

Big number bug On June 4, 1996 an unmanned Ariane 5 rocket launched by the European Space Agency exploded just forty seconds after its lift-off from Kourou, French Guiana. The rocket was on its first voyage, after a decade of development costing $7 billion. The destroyed rocket and its cargo were valued at $500 million. A board of inquiry investigated the causes of the explosion and in two weeks issued a report. It turned out that the cause of the failure was a software error in the inertial reference system. Specifically a 64 bit floating point number relating to the horizontal velocity of the rocket with respect to the platform was converted to a 16 bit signed integer. The number was larger than 32,768, the largest integer storeable in a 16 bit signed integer, and thus the conversion failed.

Pentium II bug Software bug encoded in hardware Division algorithm uses a lookup table of 1066 entries Only 1061 of the entries are downloaded to the PLA (programmed logic array from which the data are used) Intel had to recall all versions of the chip

NASA Mariner 1, Venus probe (1992) Intended to be the first US spacecraft to visit another planet, it was destroyed by a range officer on 22 July 1962 when it behaved erratically four minutes after launch. –The alleged missing `hyphen' was really a missing `bar'. –(period instead of comma in FORTRAN DO-Loop)

AT&T long distance service fails for nine hours (Wrong BREAK statement in C-Code, 1990) January 15, 1990: 70 million of 138 million long distance customers in the US lost long distance service. Cost to ATT was between $ 75 Million and $100 Million (plus the loss of good will).

buffer overflow (1998) Several systems suffer from a "buffer overflow error", when extremely long addresses are received. The internal buffers receiving the addresses do not check for length and allow their buffers to overflow causing the applications to crash. Hostile hackers use this fault to trick the computer into running a malicious program in its place.

Everything has bugs Bug lists

Summary Programming is hard –Have to thoroughly understand the task –Have to anticipate all possibilities –Code is written at a fairly primitive level –Is impossible to anticipate what users might do Programming languages allow the user to use tools to build things The cost of a bug can be very large There is no Moore’s Law for software.

Where are we We’ve built a computer We’ve looked at operating systems We’ve looked at the network We’ve built programs –And looked under the hood

What’s next One more piece of networking –Sharing files, sharing cycles, distributed computing Algorithms –Ideas of how to design processes Complexity theory –Undecidable problems –Unsolvable (in practice) problems Applications of hard problems Social impacts –Digital rights management –Artificial intelligence